1. Field of the Invention
The present invention relates to solar cells and arrays of such solar cells, and more particularly to interconnects for electrically connecting a first solar cell to a second solar cell or to a terminal in a solar cell array.
2. Description of the Related Art
Solar cell interconnects are used to electrically connect a first solar cell to a second solar cell or to a terminal in an array of solar cells. Such interconnects may be used to connect the solar cells of a solar cell array in either a parallel or a series configuration, or both. An array of solar cells interconnected in either series or parallel configuration is typically used to provide electric power to orbiting space vehicles such as satellites. One of the tasks which must be performed by an interconnect utilized in a solar cell array for an orbiting space vehicle is to absorb changes in the spacing between the cells of the array which result from severe thermal cycling encountered by orbiting spacecraft. Typically, an orbiting spacecraft is subjected to cyclical changes in temperature as great as 250.degree. C. Such mechanical or thermal cycling of the interconnects causes stress concentration in the interconnect material and in the solar cell itself where the interconnect is attached to the solar cell.
FIG. 1 illustrates a typical interconnect arrangement used in a solar cell array for orbiting spacecraft. Such a prior art interconnect, generally referred to as 10, comprises a generally semicircular central loop portion 12 and two generally flat end portions 14 and 16 extending from either end of loop portion 12. End portion 14 of interconnect 10 is permanently attached to a bottom contact 18 of a first solar cell 20, and end portion 16 of interconnect 10 is permanently attached to a top contact 22 of an adjacent solar cell 24. Frequently end portion 16 is under the solar cell cover 25, which protects the cell from radiation, and which is fixed to the solar cell 24 with transparent adhesive 26. End portions 14 and 16 of interconnect 10 are typically attached to solar cells 20 and 24, respectively, by a weld or a solder bond. The solar cells 20 and 24 are typically bonded to a substrate 70 by a silicone adhesive 71.
When this assembly is subjected to thermal cycling the substrate and cells expand and contract in accordance with the coefficient of thermal expansion of the material comprising the substrate and the cells, and the resulting change in spacing between the cells is absorbed by flexing in loop portion 12. However, such flexing in loop portion 12 creates relatively high levels of stress concentrations in the material comprising loop portion 12 due to the configuration and orientation of the loop. Such repeated stress may lead to fracturing of the interconnect itself.
During thermal cycling, the weld or solder bonds, and the cell and interconnect areas adjacent to the bonds experience stress due to the difference of coefficients of thermal expansion of these materials. This creates relatively high levels of stress concentrations where end portions 14 and 16 are connected to solar cells 20 and 24. Such repeated stress concentration may lead to fracturing of the bonds, or fracturing of the solar cell or the interconnect at the attachment to end portions 14 and 16.
Such fracturing of the interconnect or of the solar cell is obviously highly disadvantageous particularly when the solar cell is disposed in an orbiting spacecraft. Furthermore, such prior art solar cell interconnects as illustrated in FIG. 1 partially lose their ability to expand and contract with thermal cycling if the loop is filled or partially filled with solar cell adhesive as may happen when the solar cell array is being assembled. Typically, because of the size and location of loop portion 12, it is relatively easy for adhesive to enter into the area immediately below loop portion 12. If this happens, experience has shown that the interconnect will fail prematurely.
Moreover, since space is obviously at a premium in orbiting spacecraft, it is desirable to dispose the solar cells comprising the array as close to one another as possible. Such close spacing of the solar cells decreases the tolerances which must be used in manufacturing interconnect 10, and particularly loop portion 12 of interconnect 10. Such tight tolerances present problems in the fabrication of the interconnects.
Furthermore, such prior art solar cell interconnects as illustrated in FIG. 1 make removal and replacement of a broken cell difficult. If solar cell 20 is broken, the cell is typically chipped off the substrate and removed piece by piece along with interconnect 10. Before this, the glass cover on top of interconnect end portion 16 is taken off the unbroken cell 24 so that the interconnect may be desoldered from cell 24. Removing the glass cover 25 inevitably breaks the cover and sometimes breaks the cell 24.
Therefore, it is an object of the present invention to provide a solar cell interconnect having an increased life expectancy when subjected to extended periods of severe thermal cycling.
It is a further object of the present invention to provide a configuration for a solar cell interconnect which is easy to manufacture and which does not require tight tolerances in the fabrication thereof.
It is a further object of the present invention to provide a solar cell interconnect which reduces stress concentrations during severe thermal cycling in the body of the interconnect and at the attachment of the interconnect to the solar cells.
It is a further object of the present invention to provide a solar cell interconnect which is configured to facilitate the removal and replacement of solar cells by permitting one part of the interconnect to be easily disconnected from another part, and by permitting a damaged cell to be replaced without removing the cover of an adjacent cell.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.